Sawtooth voltage generator

ABSTRACT

The sawtooth voltage generator is intended in particular for use as a clock and reference voltage generator of an electronic regulating means for a motor vehicle heating system. The generator has an output voltage having a leading edge determined by a first RC circuit and a trailing edge determined by a further RC circuit. A first capacitor (C1) and a second capacitor (C3) are charged rapidly; the second capacitor (C3) has its own discharging circuit, e.g. a resistor (R7). The curve form of the trailing edge of the output voltage can be substantially freely determined with a nonlinear wave shape which is suitable for compensating for non-linear characteristics of elements of a regulating apparatus intended for receiving the output voltage with the operating frequency being determined by the discharge circuit (R4), the first capacitor (C1). The RC member (C3, R7) which determines the trailing edge of the output voltage is coupled for charging, by a diode (D2), to the first capacitor (C1) so that two capacitors (C1, C3) can be charged very rapidly, substantially via a single low-resistance charging resistor (R5).

The invention relates to a sawtooth voltage generator which isparticularly switchable as a clock and reference voltage generator of anelectronic regulation means for a motor vehicle heating system.

BACKGROUND

A sawtooth voltage generator has already been proposed, in which theoutput voltage is picked up at a capacitor which could be charged via afirst resistor of low resistance and discharged via a second resistor ofhigher resistance and with a longer time constant. The result is theequivalent of two RC circuits or members, in which the capacitor, i.e.the C member is the same for both circuits, but the resistors, i.e. theR members were different. The first RC circuit, upon charging thecapacitor, determines the leading edge; the second, or further RCmember, upon discharge of the capacitor, the trailing edge of thesawtooth voltage. This sawtooth voltage generator thus had a verysteeply rising leading edge of the sawtooth voltage because of thelow-resistance charging resistor, and thus the clock time was determinedalmost exclusively by the discharging time constant of the dischargingcircuit of the capacitor.

THE INVENTION

It is an object to provide a sawtooth wave generator in which thefrequency or repetition or cycling time of the sawtooth wave isselectable independently of the wave shape of the sawtooth wave.

Briefly, two capacitors are provided, connected to a common chargingcircuit which is of low resistance, so that the two capacitors are,effectively, rapidly and practically instantaneously charged to apredetermined charge voltage. A first one of the capacitors is connectedto a discharge circuit which includes a resistor, the discharge circuitresistor, in combination with the capacitor, defining the time constantor timing or frequency of the sawtooth wave generator. The secondcapacitor is discharged by an independent discharge circuit, decoupledfrom the first discharge circuit, for example through a second resistorwhich may well have a value differing from the first, and to a voltagelevel which may differ from that of the first, for example beingsomewhat higher. The wave shape of the second capacitor-resistordischarge circuit then may well differ from that of the first; yet, thecycling time is governed by the first. The wave shape of the dischargecircuit is derived, for example, from across the second capacitor. Thesecond capacitor cannot, however, resume a charging cycle until thefirst capacitor is completely discharged so that, regardless of thedischarge wave shape of the second capacitor, the cycling time orfrequency of operation of the sawtooth generator is determined by thefirst capacitor--first discharge resistor network.

The sawtooth voltage generator has the advantage over the prior art thatthe shape of the curve of the trailing edge can be adjustedindependently of the cycling or clock time of the generator. This curveform is variable within wide limits tending in the direction of greatercurvature, and the dimensions of the circuitry components are entirelyuncritical. A steady curve course can be obtained, without sharp bends.The trailing edge can have a substantially freely selectablenonlinearity, which can be utilized for attaining particular effects ina subsequently disposed regulating apparatus.

The sawtooth voltage generator according to the invention isparticularly suitable for use in an electronic heating regulation means,as a clock generator and reference voltage generator for the pulse-widthmodulation of an output signal. With previously known circuits insawtooth voltage generators for this purpose, it was not possible toexert notable influence on the curve form of the sawtooth voltage, yetit is necessary to do so when such a generator is used in an electronicheating regulation apparatus, in order to generate a suitablenonlinearity of the trailing edge of the sawtooth voltage so as tocompensate for the nonlinearity of an adjusting member used.Non-linearities of a heat exchanger and a hot-water valve arecompensated by this circuit. In designing an operationally reliable,simple circuit, it is particularly advantageous if the first capacitoris chargeable in a manner known per se to approximately the same voltageas the further, or second capacitor, via the same low-resistancecharging circuit. The second capacitor is discharged via a separatedischarging circuit to a higher voltage, with a substantially freelyselectable time constant. In accordance with a feature of the invention,the necessary output voltage is efficaciously generated by means of anoperational amplifier functioning as a comparator, the direct, ornon-inverting input of which is located at the connecting point of areference voltage divider connected to a direct-voltage supply sourceand the output of which is positively fed back to the non-invertinginput via an ohmic resistor on the one hand and negatively fed back viaa time-delay element e.g. a resistor on the other. An embodiment of thiskind, known per se, for the basic circuit layout according to theinvention makes it possible to design an operationally reliable andsimple circuit using only a few components. The first capacitor isdischarged in the vicinity of the end of trailing edge of the sawtoothwave, in the direction of the ground potential connected to the outputof the operational amplifier, while a voltage greater than zero isprespecified as the discharge voltage for the further capacitor. Theapparatus is advantageously designed such that the further capacitordischarges, during the discharge period, to the reference voltage of thelower switching threshold of the operational amplifier.

DRAWING

FIG. 1 shows a circuit in simple form, and

FIG. 2 is, a voltage diagram for this circuit.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

In FIG. 1, a direct-voltage supply apparatus is indicated by the symbolU_(s), which is preferably the on-board electrical system of a motorvehicle. The direct voltage is connected to one terminal of a resistorR1, the other terminal of which is connected with a secondvoltage-divider resistor R2, which in turn is connected to ground orchassis. The direct, or non-inverting input of an operational amplifier10 is connected via a resistor R6 to the junction, or connecting linebetween the resistors R1 and R2, and the inverting input of theoperational amplifier 10 is connected via a capacitor C2 to the samejunction or connecting line. The output of the operational amplifier 10is positively fed back on the one hand to the reference voltage dividerR1, R2 via a resistor R3 and is negatively fed back on the other to itsinverting input via a resistor R4 and a parallel series circuitcomprising a resistor R5 and a diode D1 in which resistor R5 hassubstantially lower resistance than resistor R4. The inverting input ofthe operational amplifier 10 is furthermore connected via a capacitor C1to ground or chassis.

The anode of a diode D2 is furthermore connected to the connecting pointor junction of the resistor R5 and the anode of the diode D1, while thecathode of the diode D2 is located at a connecting point or junction 11.The sawtooth-like output voltage U_(a) is picked up at this junction 11.One electrode of a capacitor C3, the other electrode of which isconnected to ground or chassis potential, is also connected to thisjunction 11, as is one end of a further resistor R7, the other end ofwhich is again located at the connecting line between the voltagedivider resistors R1 and R2.

The capacitor C2 at the inverting input of the operational amplifier 10serves to suppress interference voltages; the resistor R6 at thenon-inverting input of the operational amplifier 10 acts as a means ofprotection from voltage peaks.

OPERATION

When the direct-voltage supply apparatus U_(s) is switched ON, positivepotential reaches the non-inverting input of the operational amplifier10; the capacitor C1 is uncharged. On account of the voltage differenceexisting between the two inputs of the operational amplifier, themaximum output voltage, which is approximately equal to the positivesupply voltage, is present at its output, so that the capacitor C1 ischarged via the resistors R4 and R5. The resistor R5 is of substantiallylower resistance than the resistor R4, so that practically no chargingcurrent flows via R4 into the capacitor C1. At the same time, a chargingcurrent flows via the same resistor R5, at practically the same level,and via the diode D2 into the capacitor C3. This capacitor is thuscharged within the same time, to the same voltage, as is the capacitorC1; this may be seen in the area of the leading edge in the diagram ofFIG. 2.

After a very brief charging time, the difference at the two inputs ofthe operational amplifier has become so slight that the upper switchingthreshold is attained. The output of the operational amplifier jumps tothe minimal output voltage value, which is approximately equal to groundor chassis potential. From this instant on, the capacitor C1 isdischarged toward ground via the resistor R4, and the capacitor C3 isdischarged via the resistor R7 in the direction of a voltage which ishigher than zero volts.

The resistor R3 acts as a positive feedback resistor upon the referencevoltage divider R1, R2. The resistors R4 and R5 as well as the diode D1and the capacitor C1 provide the negative feedback of the operationalamplifier 10. Since the charging of the capacitor C1 is effectedparticularly rapidly via the resistor R5 of very low resistance and viathe diode D1, as may be seen from the steeply rising edge of thesawtooth voltage in FIG. 2, the clock time of the generator isdetermined virtually exclusively by the discharging time constantderived from the resistors R4 and C1.

In order to obtain a curve form of the trailing edge of the sawtoothvoltage which is freely adjustable within wide limits, the circuitincluding the resistor R7, the diode D2 and the capacitor C3 isprovided, all connected to the on-board network. The capacitor C3 ischarged, during the charging time of the capacitor C1, to virtually thesame voltage as that of capacitor C1 via the resistor R5 of lowresistance and the diode D2. The two voltage peaks in FIG. 2 thuscoincide. The charging current of the capacitor C3 flowing over theresistor R7 is negligible, as is the slight difference, dictated bymanufacturing tolerances, between the two diodes D1 and D2 which are ofthe same type. During the discharging time of the capacitor C1, thefurther capacitor C3 also discharges, but not via the resistor R4 towardthe ground or chassis potential at the output terminal of theoperational amplifier as does the capacitor C1, but rather via theresistor R7 to a voltage greater than zero volts--for instance, to thereference voltage of the lower switching threshold of the operationalamplifier. The condition here is that this lower switching threshold,represented by the resistors R1, R2 and R3, be of sufficiently lowresistance or that it be uncoupled with respect to the dischargingresistor R7, perhaps via a further operational amplifier (not shown)which functions as a voltage follower. Since the discharging timeconstant for the layout comprising the resistor R7 and the capacitor C3is adjustable entirely independently of the time constant of the memberwhich determines the clock time and which is made up of the resistor R4and the capacitor C1, a sawtooth voltage having virtually any arbitrary,sharply pointed curve form or a more sharply bent curvature than at thecapacitor C1 can be generated at the capacitor C3. The voltage droppingat the capacitor C1 is indicated in FIG. 2 by dashed lines and clearlyshows the difference in the nonlinearity of the curves. By means of thecircuit layout according to the invention, it becomes possible to makethe curve form of the trailing edge of the sawtooth voltage freelyadjustable, in accordance with the desired nonlinearity, so as tocompensate for other nonlinearities of a regulating apparatus.

Alternative circuitry to the operational amplifier 10 may be embodied insuch a manner that the amplifier is made up of discrete transistors.

I claim:
 1. A sawtooth voltage generator, particularly for use as aclock and a reference voltage generator for an electronic regulatingapparatus for a motor vehicle, havinga source of voltage (U_(S)); means(10) coupled to the voltage source and providing a charging voltage; andcomprising a first capacitor (C1); a second capacitor (C3); a commonlow-resistance charging resistor (R5) connecting the charging voltageproviding means (10) to said first and second capacitors (C1, C3) tocharge said capacitors to substantially the same charge voltage; a firstdischarge circuit (R4) coupled to the first capacitor (C1) and to areference voltage, and defining, with said first capacitor, a firstdischarge time constant; a second discharging circuit (R7) coupled tothe second capacitor and to a voltage which differs from said referencevoltage level, and defining, with said second capacitor, a seconddischarge time constant; means (D1, D2) effectively decoupling saidfirst and second discharge circuits from each other and from said commoncharging resistor (R5); and means deriving a sawtooth output voltagewave (Ua) having a wave form representative of the discharge wave form(U_(C) 3) of said second capacitor (C3) and a time constant depending onthe longest one of the discharge time constants, to permit freelyselecting the time constant of the sawtooth output voltage wave (U_(A))and freely selecting the wave form of the sawtooth output voltage wavebased on a discharge time constant of the second discharge circuit (R7)for the second capacitor, while permitting maintaining the cycling timeconstant or frequency of operation of the sawtooth output voltage wave(Ua) of the first discharge circuit (R4) of the first capacitor (C1). 2.Sawtooth voltage wave generator as defined in claim 1, wherein saiddecoupling means comprises two diodes (D1, D2) respectively connectedand poled to permit charging current to flow from said commonlow-resistance charging resistor (R5) to, respectively, the firstcapacitor (C1) and the second capacitor (C3), while decoupling thedischarge circuits (R4, R7) for said capacitors from each other. 3.Sawtooth voltage wave generator as defined in claim 1, furthercomprising an operational amplifier (10) connected as a comparator, andhaving a direct input and an inverting input;a voltage divider (R1, R2)connected between the voltage source (U_(s)) and reference potential,the voltage divider having a common junction or tap point, the commonjunction being connected to the direct input of the operationalamplifier; a positive feedback connection (R3) from the output of theoperational amplifier to the common junction or tap point of the voltagedivider; and a negative feedback connection (R5, R4, D1) from the outputof the operational amplifier to the inverting input thereof, saidnegative feedback connection including the first discharge circuit (R4)and being coupled to said first capacitor (C1).
 4. Sawtooth voltage wavegenerator as defined in claim 3, wherein said second discharge circuitcomprises a resistor (R7) coupled to the common junction or tap point ofthe voltage divider.
 5. Sawtooth voltage wave generator as defined inclaim 1, wherein the voltage to which the second discharge circuit (R7)discharges the second capacitor (C3) is at a level higher than saidreference level.
 6. Sawtooth voltage wave generator as defined in claim1, wherein said first and second discharge circuits, each, compriseseparate resistors (R4, R7);and the decoupling means comprise diodes(D1, D2) poled and connected to permit charging current to flow fromsaid common low-resistance charging resistor (R5) to the first capacitor(C1) and to the second capacitor (C3), respectively, but blockingdischarge current from flowing therethrough to permit discharge of therespective capacitors through the respective discharge resistors (R4,R7).
 7. Sawtooth voltage wave generator as defined in claim 6, furthercomprising an operational amplifier (10) connected as a comparator, andhaving a direct input and an inverting input;a voltage divider (R1, R2)connected between the voltage source (U_(s)) and reference potential,the voltage divider having a common junction or tap point, the commonjunction being connected to the direct input of the operationalamplifier; a positive feedback connection (R3) from the output of theoperational amplifier to the common junction or tap point of the voltagedivider; and a negative feedback connection (R5, R4, D1) from the outputof the operational amplifier to the inverting input thereof, saidnegative feedback connection including the first discharge circuit (R4)and being coupled to said first capacitor (C1).
 8. Sawtooth voltage wavegenerator as defined in claim 7, wherein the voltage to which the seconddischarge circuit (R7) discharges the second capacitor (C3) is at alevel higher than said reference level.
 9. Sawtooth voltage wavegenerator as defined in claim 7, wherein said second discharge circuitcomprises a resistor (R7) coupled to the common junction or tap point ofthe voltage divider.